1. Field of the Invention
The present invention relates to feed-through capacitors having a structure comprising a feedthrough terminal and an outer electrode terminal surrounding through a dielectric layer which in turn surrounds the feedthrough terminal.
2. Description of the Prior Art
In a magnetron in a microwave oven or the like, a filter circuit is generally inserted into a power supply line so as to prevent a microwaves from leaking as noise via the power supply line or the like. Two conventional types of feed-through capacitors used in this type of filter circuit are respectively shown in FIGS. 9 and 11.
Each of the feed-through capacitors is constructed by using a dielectric block made of ceramics obtained by molding a ceramic dielectric material having a high dielectric constant in a block shape and sintering the same.
A dielectric block 4 of a feed-through capacitor 1 is shown in a cross-sectional view in FIG. 9 and is shown in perspective in FIG. 10. This dielectric block 4 is approximately elliptical in cross section and has two through holes 2 and 3 penetrating from its upper end surface to its lower end surface. Electrodes 6a and 6b separated by a slit-shaped groove 5 are formed on the upper surface of the dielectric block 4, while an electrode 7 is formed on the lower end surface thereof. The electrodes 6a and 6b and the electrode 7 are opposed to each other, two capacitances being formed therebetween.
Connecting terminal boards 11 and 12 are respectively soldered to the electrodes 6a and 6b formed on the dielectric block 4, as shown in FIG. 9. A hole 11a is formed in the connecting terminal board 11. A feedthrough terminal 13 is inserted into the hole 11a and is soldered or welded to the inner peripheral surface of the hole 11a. As obvious from FIG. 9, the feedthrough terminal 13 is inserted into the through hole 2 with it not being in contact with the inner peripheral surface of the through hole 2 of the dielectric block 4.
Similarly, the other feedthrough terminal 14 is inserted into a hole 12a formed in the other connecting terminal board 12 and is soldered or welded to the inner peripheral surface of the hole 12a.
A ground terminal board 16 is soldered to the electrode 7 on the lower end surface of the dielectric block 4. The ground terminal board 16 has a hole 16a for pulling out the feedthrough terminals 13 and 14 to pass downward. The electrode 7 is soldered to the outer peripheral surface of the hole 16a. An insulating case 17 and another insulating case 18 are respectively arranged above and the below the connecting terminal board 16. Insulating resin 19 is filled in the insulating cases 17 and 18.
A feed-through capacitor 21 shown in FIG. 11 is constructed by using a dielectric block 25. In the dielectric block 25, electrodes 22 and 23 are formed on the inner peripheral surfaces of through holes 2 and 3 in which feedthrough terminals 13 and 14 are respectively inserted. In addition, an electrode 24 is formed on the outer side surface of the dielectric block 25 so as to be opposed to the electrodes 22 and 23.
The electrodes 22 and 23 in the through holes 2 and 3 and the electrode 24 formed on the outer side surface of the dielectric block 25 are opposed to each other with the dielectric block 25 being interposed therebetween, a capacitance being formed therebetween.
The feedthrough terminals 13 and 14 are directly soldered to the electrodes 22 and 23 in the through holes 2 and 3 of the dielectric block 25 by solder 26. In addition, the dielectric block 25 is inserted into a hole 27 provided in a ground terminal board 16. The electrode 24 formed on the outer side surface of the dielectric block 25 is soldered to the ground terminal board 16.
Insulating cases 17 and 18 are respectively arranged on one side and the other side of the ground terminal board 16. Insulating resin 19 is filled in the insulating cases 17 and 18.
The feed-through capacitors constructed by using the dielectric blocks 4 and 25 obtained by molding the ceramic dielectric material in a block shape and sintering the same as described above can easily satisfy the following performance requirements for a filter circuit in the magnetron in the microwave oven.
(a) filter performance; to absorb noise in a frequency band of the television or radio wave (30 to 300 MHz) and in the microwave range (1 GHz or higher) and prevent conduction and radiation to the exterior.
(b) voltage proof performance; to withstand a rush pulse and a rise pulse of 10 to 20 kV.sub.O-P at the time of the oscillation of the magnetron.
In particular, the filter performance described above in
(a), a requires real capacitance of 100 pF or more in the frequency range of a television or radio wave, which is easily satisfied by using a ceramic dielectric body. In addition, the noise in the microwave band is mainly radiation noise. However, the use of the ceramic dielectric body makes it easy to damp the radiation noise.
In the conventional feed-through capacitors using the dielectric blocks 4 and 25 obtained by molding the ceramic dielectric material in a block shape and sintering the same, however, the shapes and the structures of the dielectric blocks 4 and 25 are complicated. Consequently, the feed-through capacitors have the disadvantage in that the fabrication of the dielectric blocks 4 and 25 having such complicated shapes and structures is very troublesome, resulting in increased cost.
Furthermore, in the above described feed-through capacitors, high voltage performance is also required. Accordingly, the outer peripheral surfaces of the dielectric blocks 4 and 25 are molded and enclosed by insulating resin of the epoxy resin system 19. However, the coefficient of linear expansion .alpha. and the modulus of elasticity E of the dielectric blocks 4 and 25 are significantly different from those of the feedthrough terminals 13 and 14 and the insulating resin 19. Consequently, the conventional feed-through capacitors 1 and 21 also have the disadvantages in that the insulating resin 19 and the dielectric blocks 4 and 25 can be, for example, crazed, cracked or stripped off in performing a test of a thermal cycle such as a heat cycle.